Polyimides, including polyimide sulfone and polyetherimide sulfone resins, are known to be useful high performance polymers. A desired process for producing polyimide sulfone resins through a heterogeneous process is disclosed in U.S. Pat. No. 7,041,773 B2 to Gallucci et al., which results in resins having a total reactive end group concentration of less than about 120 milliequivalents/kilogram (meq/kg) resin. The polyimide sulfones may be made in a solvent process. Typically, polymerization in a solvent takes place at temperatures of about 100° C. to about 250° C. Solvents with a boiling point of greater than about 150° C. at atmospheric pressure are used allowing the polymerization reaction to be carried out at higher temperature which improves the reaction rate. The higher temperature also typically allows the polymer to remain partially in solution, thus facilitating the build of higher molecular weight. Higher molecular weight polymer often gives better mechanical properties in articles formed from polyimide sulfone resins.
To control the stoichiometry to produce desired reactive end group concentration in the final resin, the concentration of end groups on the polyimide sulfone product is measured in the reaction mixture at various times during synthesis of the polyimide sulfone. For example, the concentration of end groups can, be measured at a specific time, or when the polymer has been determined to have reached some molecular weight or viscosity target, or when the water being removed from the reaction is observed to diminish or essentially stop. If the analysis shows that the total concentration of reactive end groups is greater than 120 meq/kg, then at least one of aromatic diamine monomer or aromatic dianhydride monomer or primary monoamine capping agent or aromatic dicarboxylic acid anhydride capping agent may be added to the reaction mixture to reduce the total concentration of reactive end groups to 120 meq/kg or less and in the appropriate amount to provide the desired molecular weight of the polymer. The steps of analyzing the polymer in the reaction mixture for end group concentration and optionally adding an amount of at least one of monomers or capping agents to the reaction mixture to reduce reactive end group concentration may be performed as many times as necessary to provide a reactive end group concentration at or below 120 meq/kg in the final polymer product and achieve the desired molecular weight.
While the process described produces the desired result, the type of material and amount of material to be added during the reaction is determined by direct measurement of reactive end groups in solution by using a sample that can be totally dissolved in solvent to facilitate direct reactive end group measurement. The process time may need to be extended to allow for further correction if the incorrect material or incorrect amount of material is added. Continuing correction is needed until the desired product is obtained. Further, an error in judgment as to what and how much to add to the reaction may result in the process producing an undesired product. In the heterogeneous case or a case where the sample cannot be dissolved, the error is compounded by the partitioning of reactive species (monomers, oligomers and polymers) in various phases of solid and solvent. Thus, there remains a need to develop a process with more precision such that the addition of materials to correct stoichiometry of the polymerization reaction produces the desired results without speculation. It is to the provision of such that the present invention is primarily directed.